The present invention is directed to a method and apparatus for an autonomous navigation system for estimating a set of satellite navigation parameters based on measurements from Global Positioning System (GPS) and magnetometers.
GPS has become a standard method for low cost onboard satellite orbit determination. The use of a GPS receiver as an attitude and rate sensor has also been developed in the recent past. GPS is emerging as the favored technique that provides sufficient accuracy for most requirements.
However, GPS requires expensive gyroscopes (gyros) or star trackers to be fully functional and provide accurate attitude data. In addition, GPS is subject to initialization delays and signal drop-outs that require back up systems to fill in during the loss of GPS data.
This invention presents a method and apparatus for processing GPS phase, pseudo-range, and range rate data as well as magnetometer data to produce estimates of attitude, angular rate, position, and velocity for a spacecraft, especially a low earth orbit (LEO) satellite.
Magnetometers are a low cost and reliable technology that have been used to determine satellite orbit and attitude based on the earth""s magnetic field. Magnetometers always output data as they are not subject to occultation or tracking problems.
Although GPS is relatively a low-cost sensor, providing accurate orbit estimates, with coarse attitude estimates, GPS based systems are subject to initialization problems, and must rely on costly systems such as gyros. However, GPS data is available continuously throughout the orbit and can produce more accurate orbit, attitude, and rate estimates by combining it with magnetometer data.
The present invention solves the drawbacks associated with GPS-based systems as mentioned above, by coupling GPS with reliable and low cost magnetometers. By processing the orbit and attitude solutions in the same algorithm in a single self-contained unit, weight, size, and power consumption are reduced. Completely autonomous satellite navigation is thus possible with reduced weight and cost.
The method used is an extended Kalman filter (EKF), blended with a pseudo-linear Kalman filter algorithm, developed to provide estimates of attitude, orbit, and rate estimates using magnetometer and GPS data. The magnetometer based EKF can converge from large initial errors in orbit, attitude, and rate. Combining the magnetometer and GPS data into a single EKF provides a more robust and accurate system that produces estimates of attitude, angular rate, position, and velocity for a spacecraft.
Some publications listed, relating to the present invention, are incorporated by reference:
1. Deutschmann, J., R. Harman, and I. Bar-Itzhack, xe2x80x9cAn Innovative Method for Low Cost, Autonomous Navigation for Low Earth Orbit Satellitesxe2x80x9d, Proceedings of the AAS/AIAA Astrodynamics Specialists Conference, Boston, Mass., Aug. 10-12, 1998.
2. Oshman, Y, and F. L. Markley, xe2x80x9cSpacecraft Attitude/Rate Estimation Using Vectorxe2x80x94Aided GPS Observationsxe2x80x9d, IEEE Transactions on Aerospace and Electronic Systems, Vol. 35, No. 3, July 1999.
3. Deutschmann, J., and I. Bar-Itzhack, xe2x80x9cComprehensive Evaluation of Attitude and Orbit Estimation Using Real Earth Magnetic Field Dataxe2x80x9d, Proceedings of the 11th Annual AIAA/USU Conference on Small Satellites, Logan, Utah, Sep. 15-18, 1997.
4. Bar-Itzhack, I., xe2x80x9cClassification of Algorithms for Angular Velocity Estimationxe2x80x9d, Journal of Guidance, Dynamics, and Control, Vol. 24, No. 2, March-April 2001.
5. Deutschmann, J., R. Harman, and I. Bar-Itzhack, xe2x80x9cA LEO Satellite Navigation Algorithm Based on GPS and Magnetometer Dataxe2x80x9d, Proceedings of the CNES 15th International Symposium on Space Flight Dynamics, Biarritz, France, Jun. 26-30, 2000.
6. Brown, Robert Grover, and Patrick Y. C. Hwang, Introduction to Random Signals and Applied Kalman Filtering, (3rd ed.), John Wiley and Sons, 1997.
7. Wertz, James. R, editor, Spacecraft Attitude Determination and Control, D. Reidel Publishing Company, 1984.
8. Shorshi, G. and I. Bar-Itzhack, xe2x80x9cSatellite Autonomous Navigation Based on Magnetic Field Measurementsxe2x80x9d, Journal of Guidance, Control, and Dynamics, Vol. 18, No. 4, July-August, 1995.